1. Field of the Invention
The present invention relates to an emulsion ink for use in stencil printing, particularly to a W/O emulsion ink which is applicable to a rotary press of central cylinder type.
2. Discussion of Background
In the stencil printing method, an emulsion ink for use in stencil printing is applied to one surface of a stencil paper which is prepared by perforation in accordance with the desired original images, and the emulsion ink applied to a perforated portion of the stencil paper is allowed to pass through the stencil paper, thereby achieving the print of images on the surface of an image-receiving medium such as a sheet of paper.
In recent years, in line with the trend toward simple mechanism and compact size of the printing system, the dual drum type press has been switched over to the central cylinder type press in the stencil printing. The rotary press of the dual drum type is designed not to supply the surface of the stencil paper with an excessive amount of ink, so that a soft ink can be used for printing. With the rotary press of central cylinder type, however, there is the problem that a press roll or a printed matter becomes stained with the ink flowing to the end portion of a master plate when a soft ink is used for printing.
In addition, a high boiling solvent such as a non-drying oil, or a nonvolatile mineral oil, for example, motor oil, liquid paraffin, spindle oil, machine oil or castor oil is contained in the conventional ink composition so as to prevent the evaporation of the ink composition while it is allowed to stand on the cylinder. This means the prevention of clogging of the perforated portions of the stencil paper after printing. However, when the conventional W/O emulsion ink composition is allowed to stand in the stencil printing press, only the water content evaporates. The balance between the water content and the oil content is destroyed, and the amount ratio of the oil content is relatively increased. As a result, the viscosity of the ink composition is decreased, and the ink is softened. The density of printed images becomes excessively high, and therefore, an offset phenomenon and blurred images are caused.
There are many proposals to solve the above-mentioned problems. For example, Japanese Laid-Open Patent Application 61-255967 discloses a W/O emulsion ink for stencil printing, comprising an oil phase and a water phase at the ratio by wt. % of (10 to 50):(90 to 50), with the oil phase containing a solvent having a boiling point of 180 to 270.degree. C. in an amount of 5 wt. % or more, preferably 10 wt. % or more of the entire weight of the water phase.
In the above disclosure, however, the medium-boiling solvent having a boiling point of 180.degree. to 270.degree. C. substantially accounts for 30 to 100 wt. % of a solvent component in the oil phase. When the stencil printing press is allowed to stand for a long period of time with being supplied with the above-mentioned emulsion ink, the medium-boiling solvent in the oil phase and the water content in the water phase evaporate. Therefore, when the amount ratio of a nonvolatile solvent contained in the emulsion ink remaining on a drum of the stencil printing press is small, a resin component contained in the emulsion ink is completely or nearly hardened. Such a hardened resin component adheres to a drum mesh to clog it up, or adheres to an ink roller to make the ink roller immovable.